Technical Field
The present invention relates to a hydraulic drive apparatus for moving a load such as a suspended load in the same direction as a self-weight falling direction, in which the load falls by its own weight, in a working machine such as a crane.
Description of the Background Art
As an apparatus for moving a load in the same direction as its self-weight falling direction, there is known a lowering drive apparatus for driving, for example, a winch for hanging a suspended load by a wire in a lowering direction. In this apparatus, it is important to prevent the suspended load from falling down due to stall resulting from a drop in meter-in side pressure which causes cavitation during lowering drive.
As a means for preventing such a drop in the meter-in side pressure, it is disclosed in Japanese Unexamined Patent Publication No. 2000-310201 to provide so-called an external-pilot-controlled counter balance valve in a meter-out side flow passage. This external-pilot-controlled counter balance valve is operated to choke the meter-out side flow passage when the meter-in side pressure falls to or below a set pressure, thereby preventing the meter-in side pressure from excessive drop.
The control by the external-pilot-controlled counter balance valve, however, has a problem of being inherently unstable and prone to hunting, because respective positions of the measurement point and the control point are different from each other, specifically, having its pressure measurement point on the meter-in side while having its pressure control point on the meter-out side; thus, so-called co-location is not present in control theory.
To prevent the hunting, there can be provided such an orifice as to apply considerable damping to a valve opening operation of the counter balance valve in a pilot fluid passage; however, this orifice extends a valve opening time of the counter balance valve to thereby degrade the responsiveness of the counter balance valve. Furthermore, the orifice generates large orifice resistance in the counter balance valve until the full open of the valve, thus generating an unnecessary boost pressure.
To prevent the hunting, the above Japanese Unexamined Patent Publication No. 2000-310201 also discloses a communicating valve for allowing communication between the meter-in side flow passage and the meter-out side flow passage and a flow rate regulating valve for controlling a meter-in flow rate so as to decrease a differential pressure between the both flow passages; however, this technique involves a difficulty of obtaining a stable lowering speed. Specifically, in a general lowering control circuit, there occurs a holding pressure corresponding to the weight of a suspended load at the meter-out side, which increases a differential pressure between the meter-in side and the meter-out side with an increase in the weight of the suspended load. This increase in the differential pressure involves an increase in the opening of the flow rate regulating valve on the meter-in side, thereby increasing the meter-in flow rate. Thus, in this apparatus, the lowering speed is largely varied depending on the weight of the load.
On the other hand, there can be cases where a hydraulic actuator for moving a load in a lowering direction such as during the lowering drive is required to be arranged with another hydraulic actuator in series between a hydraulic pump and a tank to thus allow these hydraulic actuators to be driven by the common hydraulic pump.